TECHNIQUE
CONVERSIONS - Rules of “Thumb”
ONE METHOD TO CALCULATE NEW MAS for Grids & Screens
GRIDS
Bushong’s Table
Non Grid =
5:1
=
8:1
=
12:1
=
16:1
=
1X
2X
4X
5X
6X
( Carlton’s Table is different)*
* These numbers change slightly from Author to Author
GRID CONVERSIONS
This formula is used to determine the conversion rate from no grid to grid:
GCF -grid conversion factor :
GCF=
mAs without the grid
mAs with the grid
This formula is used when changing from one grid ratio to another:
mAs 1 = GCF 1
mAs 1 is the original mas,
GCF 1 is the original Grid
mAs 2
GCF 2
mAs 2 is the new mas,
GCF 2 is the new Grid used
(Note they are directly proportional)
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FILM/ SCREEN COMBINATIONS
This formula is used when changing from one film/screen speed to another:
RS = SPEED OF SCREENS IN THE FILM/SCREEN CASSETTE
This may also be known as F/S for film/screen combination
mAs 1
mAs 2
=
RS 2
RS 1
(Note they are inverse)
mAs 1 is the original mas,
mAs 2 is the new mas,
Can be otherwise written as:
RS 1 X mAs 1
mAs 2 = ------------------------RS 2
RS 1 is the original screen speed used
RS 2 is the new screen speed used
======================================================
RULES OF ORDER FOR CONVERSION
1. MAS
2. GCF
3. KVP
4. RS (F/S) FILM/SCREEN SPEED
5. SID
(Suggested – any order is okay)
TECHNICAL “RULES OF THUMB”
TISSUE THICKNESS For every 1cm of change in thickness, add or subtract 2 kvp
For every 5 cm of change in thickness, either double or half the mAs.
CENTRAL RAY ANGLE Decrease SID by 1" for every 5  tube 
MAS Changes -at least 20 - 30 % mas change needed to see a visible change in density
Maintaining Density LAW L when changing SID - mAs Changes
Above 40 “ = increase mAs 5% for every 1" increase in SID
mAs 1 = D 12
mAs 1 is the original mas, D 1 is the original distance
mAs 2
D 22
mAs 2 is the new mas,
D 2 is the new distance
(Note they are directly proportional- distance increases – mas should increase
** When changing SID from 40" to 72" = increase mAs X 3.24 (ave 3 MAS)
When changing SID from 72" to 40" =( divide) mAs  3.24
( example : KUB - 40mas at 40 “ = 129.6 mas at 72")
Use mas/distance formula to verify
GENERAL RULES FOR DISTANCE CHANGE:
When changing from 40" SID to:
New Distance
Technique Change (MAS)
30"
½
40"
1 (standard)
60"
2X
72"
3X
80"
4X
96"
6X
Remember : 15% change ( ) KVP has the same effect as doubling or ½ the MAS
INCREASE DENSITY BY 2 X
Go up 2 X in mas
or Go up 15% Kvp
you could:
Example: 20 mas - 70 kvp
40 mas - 70 kvp
20 mas - 81 kvp
INCREASE DENSITY BY 4 X you could:
Go up 4 X in mas
or Go up 2 X MAS & 15% Kvp
or
Go up 30% in KVP (15% x 2)
Example: 20 mas - 70 kvp
80 mas - 70 kvp
40 mas - 81 kvp
20 mas - 91 kvp
Maintaining Density when changing OID - Increase mAs 10% for every 1" increase in OID
FOR CHEST AND THORAX AREA - TO GO FROM AP/PA TO LAT:
* Double the MAS and go up 10 kvp